In order to meet the requirements of the information equipments, the demand for the flat panel display (FPD) is increasing day by day. In addition, since the simplification, compactness, lightness, and power-saving are the current trends for the electric equipments, the cathode ray tube (CRT) display is gradually replaced by the FPD. Nowadays, the technologies for the FPD include the following, the plasma display, the liquid crystal display, the electroluminescent display, the vacuum fluorescent display, the field emission display, the electrochromic display, and the organic light emitting diode (OLED) display. In which, compared with other display technologies, the OLED display has the following advantages, such as self-luminescing, super-thin appearance, high brightness, high luminance efficiency, short response time, wide visual angle, power saving, wide temperature tolerance, flexible panel, and so forth. Therefore, the OLED display is believed to be the major trend of the display market for the coming generation.
The principles of how the OLED luminesces are as follows. First, an organic film is formed between the transparent anode and the metal cathode by evaporating. Then, electrons and electric holes are supplied therein. Finally, through the reaction between the electrons and the electric holes, the energy is generated and transformed into the visible light. Furthermore, since the material used in the manufacturing of the OLED determines the color of the formed visible light, the requirements of the full-color display can be easily achieved. Presently, the OLED displays manufactured by applying the above principles can be divided into two types, the active matrix organic light emitting diode (AMOLED) display and the passive matrix organic light emitting diode (PMOLED) display, based on the driving method.
As shown in FIG. 1, it is common to drive a PMOLED display by using the row scan technology, which applies the three-phase driving method. In which, the OLED matrix 10 is composed of the plural rows of OLEDs and the plural columns of OLEDs and its driving method is discussed as follows.
(1) For each row of the OLEDs, two kinds of operating phases are provided: a. Current sinker phase, the current is sunk into the OLEDs successfully. b. Reverse bias phase, the object is to increase the durations of the OLEDs.
(2) For each column of the OLEDs, each segment of the OLEDs is driven by a constant current source (not shown). Each segment of the OLEDs is driven by the following three-phase driving method: a. Dis-charge phase, the retaining electric charges of the OLEDs are discharged. b. Pre-charge phase, the potentials of the OLEDs are precharged to the turn-on potentials for performing the best electrifying efficiency when current is sunk in. c. Current driving phase, the current is sunk into the OLEDs to make it to luminesce.
Please refer to FIG. 1, again. In which, the OLED matrix 10 under the conventional passive matrix phase achieves the displaying function by means of the above cyclic charging and discharging processes. However, since the OLED has a parasitic capacitance, which is an inherent physical property, it is not a perfect LED.
Please refer to FIG. 2, which shows a diagram of a practical equivalent circuit of organic light emitting diode according to prior art. In which, both the ideal LED 20 and the parasitic capacitance 21 are shown. It is obvious that the existence of the parasitic capacitance 21 will directly affect the conducting rate of the driving circuit. In other words, the OLED will not be able to perform the expected brightness when the voltage difference between the cathode and the anode of the OLED can not reach to the suitable driving voltage rapidly.
Owing to the existence of the capacitance, the above three-phase OLED array driving method will have the following two problems, which seriously affect the displaying quality. FIGS. 3–5 describes the problems. For the convenience of the description, the capacitance symbol in the drawings represents the OLED (since describing the influence of the capacitance is the object herein), and only three OLEDs within one column of the whole OLED array are shown. The above-mentioned description can be easily understood and can be expansively applied to the whole OLED array by one skilled in the art.
Please refer to FIG. 3, which is a diagram illustrating the first problem resulted from using the conventional three-phase OLED array driving method. When the dis-charge phase initiates, the cathode and anode of the OLED 30 are grounded for being discharged, but the OLEDs 31 and 32 respectively located in the second and third row are under reverse bias phase for being reverse charged. When the pre-charge phase initiates, all the OLEDs 30, 31, 32 are charged by the pre-charge power 33. In which, the voltage of the OLED 30 is charged from 0 to Vpre (a pre-charge voltage value), and the voltages of the OLEDs 31 and 32 are charged from −Vrev (a reverse bias voltage value) to Vpre. From the above, it's known that during the pre-charge phase, the electricity consumed by the OLEDs 31 and 32, which are not necessary to be driven for luminescing, are much more than that consumed by the OLED 30, which is necessary to be driven for luminescing. Therefore, under the above situation, the charging efficiency for each segment during the pre-charge phase is always low. Furthermore, under the passive matrix phase, the row number of the OLED being luminesced is always 1, and the row number of the other OLEDs is N-1, in which the letter N is assumed as the total duty numbers. As mentioned in the above, when the row number of the display panel increases, the charging efficiency of the pre-charge phase is getting worse.
Please refer to FIG. 4, which is a diagram showing the measured wave according to FIG. 3. From the wave obtained practically from the measuring, it's found that the charging process during the pre-charge phase is not operated smoothly. The efficiency of the pre-charge phase will affect the efficiency of the next phase, the current driving phase. In addition, as shown in FIG. 4, the rising time for conducting the OLED is too long.
(2) Please refer to FIG. 5, which is a diagram illustrating the second problem resulted from using the conventional three-phase OLED array driving method. When the row scan is shifted from the first row to the second row, the segments involved in the current driving phase will be switched to be dis-charge phase. In which, the anodes of the OLEDs 50, 51 and 52 are charged to a high potential during the current driving phase. Therefore, at the beginning instant of the dis-charge phase, although the anode potential of the OLED 52 is 0, a voltage drop still occurs to the cathode potential of the OLED 52, owing to the existence of the inherent capacitance effect. Then, the cathode potential of the OLED 52 can start to be charged to Vpre. This problem indeed affects the displaying quality and the situation will be getting worse if the column number of the OLEDs increases. The diagram showing the wave obtained practically from the measuring is shown in FIG. 6.